1. Field of the Invention
The present invention relates to a semiconductor light-emitting device such as a refractive index waveguide laser or a double heterojunction light-emitting diode and a method of manufacturing the same and, more particularly, to a semiconductor light-emitting device in which an active region is surrounded by a semiconductor layer whose forbidde band width is larger than that of the active region and a method of manufacturing the same.
2. Description of the Related Art
Recently, various semiconductor light-emitting devices having a double heterostructure have been developed. In the semiconductor light-emitting devices of this type, it is very important to satisfy the following conditions A to C.
A. In order to improve light-emitting efficiency, a current must be flowed effciently to only a light-emitting region or active region having a size controlled to be very small.
B. In order to reduce contact resistance, an electrode must be formed throughout a wide region.
C. When high-speed modulation must be performed as in a light-emitting device for optical communication, an area of a p-n junction is minimized in order to reduce a junction capacitance.
A known example of a semiconductor light-emitting device for optical communication which more or less satisfies the above three conditions is a mesa laser utilizing a mass transport technique (e.g., Y. Hirayama et al. "Low Temperature and rapid mass transport technique for GaInAsP/InP DFB lasers", Inst. Phys. Conf. Ser. No. 79: Chapt 3 Paper presented at Int. Symp. GaAs and Related Compounds Karuizawa, Japan, 1985 p. 175, 186). Such a semiconductor light-emitting device is called an MT laser. A method of manufacturing the MT laser and characteristics of the MT laser will be described below with reference to the accompanying drawings.
FIGS. 1A to 1D are sectional views schematically showing manufacturing steps of a conventional MT laser.
First, as shown in FIG. 1A, 3-.mu.m thick n-type InP buffer layer 2, 0.1-.mu.m thick undoped GaInAsP active layer 3 having a composition which enables emission of light having a wavelength of 1.3 .mu.m, 1.5-.mu.m thick p-type InP clad layer 4, and 0.8 .mu.m thick p.sup.+ -type GaInAsP cap layer 5 which emits light having a wavelength of 1.15 .mu.m and enables good ohmic contact are sequentially crystal-grown on n-type (100) InP substrate 1.
Then, as shown in FIG. 1B, layers 4, 5 are mesa-etched so that active layer 3 is exposed and a mesa strip having width of 15 .mu.m is formed, which allows relatively easy mask alignment. At this time, when hydrochloric acid is used to remove InP layer 4, etching can be automatically stopped at active layer 3 by selectivity of the acid.
Thereafter, as shown in FIG. 1C, both sides of active layer 3 are etched by an etchant consisting of sulfuric acid+hydrogen peroxide+water (4:1:1) to form an active region having a width of about .mu.m. At this time, InP is almost no etched, and only GaInAsP is etched. Although cap layer 5 is also etched, it is etched to a depth only about 1/3 that of active layer 3 due to a difference in composition. In order to obtain fundamental transverse oscillation and a low oscillation threshold current, the width of active layer 3 must be precisely controlled to be around 1 .mu.m.
Then, as shown in FIG. 1D, in consideration of light leakage in a transverse mode and mechanical strength, an InP layer is buried in the deep gap of active layer 3 formed by etching, thereby forming a so-called buried heterostructure (BH). In the MT laser, the MT technique is used to bury and grow the InP layer. That is, InP is grown faster in the gap portion when phosphorus is doped at a high temperature (670.degree. C.) and high pressure. Note than when InCl.sub.3 is used as an additive agent, InP can be rapidly grown at a lower temperature.
SiO.sub.2 film 6 is formed as an insulating film throughout the entire upper surface of the above element, and a window is formed in a contact portion of this insulating film. Then, AuZn is formed as p-side electrode 7 by a lift-off technique and heated to perform alloying. Au-Cr is deposited on electrode 7 and insulating film 6 to form electrode 8. N-side electrode 9 is formed on substrate 1, thereby completing the MT laser.
The above MT laser can concentrate a current at active region 3 by a difference between built-in potentials of GaInAsP of active region 3 and In of the buried portion. Since a junction is formed only at the mesa portion, a junction capacitance is small, and therefore high-speed response can be advantageously obtained. Furthermore, the width of electrode 7 can be formed to be about 10 .mu.m.
However, in the MT laser of this type, controllability of the width of the active region poses a problem. That is, when selective etching is performed from the both ends of the active layer having a width of 15 .mu.m to form an active region having a width of 1 .mu.m, it is difficult to precisely stop etching to obtain a width of 1 .mu.m. Therefore, the entire active layer is sometimes etched, resulting in poor manufacture yield. Accuracy of etching is degraded as a mesa width is increased. For this reason, the mesa width cannot be set to be 15 .mu.m or more. In addition, in consideration of mask alignment margin, the width of the ohmic electrode portion must be 10 .mu.m or less. For this reason, the contact resistance cannot be sufficiently reduced. Moreover, an area of the InP junction of the buried portion is defined by the width of the mesa portion and therefore cannot be controlled to be narrower than that. Since even if the active region may be formed to have a desired marrow width, the mesa region must be mechanically supported in the active layer having a narrow width as described above, distortion is concentrated in the active layer. Particularly, insulating film 6 has a different coefficient of thermal expansion from that of the semiconductor, resulting in the cause for distortion and stress. Therefore, the manufacture yield and reliability are degraded.
Note that alrthough the area of the buried portion can be adjusted by controlling a time of the MT step, its controllability is very poor. For this reason, the width of the buried InP junction portion cannot be reduced to reduce the junction capacitance, thereby seriously interfering with realization of high performance. In order to reduce the junction capacitance and to increase a rise voltage in the junction portion so as to reduce current leakage, a carrier density in the buried junction portion must be optimized. However, since the carrier density is not controlled in the conventional MT technique, the density in the junction portion cannot be defined, resulting in a serious problem in design.
As described above, in the conventional MT techtique, it is difficult to precisely set the width of the active region, and therefore high performance of the buried semiconductor light-emitting device is significantly interfered. In addition, when the area of the buried portion is reduced, the contact area is reduced to increase the contact resistance. When the contact area is increased, the buried area is increased to increase the junction capacitance, and it is difficult to control the width of the active region.